Cryogenic insulating panel system

ABSTRACT

A thermal jacket for a cryogenic container, the jacket being formed by a system of interfitting modular panels attachable to the wall of the container. Each panel is constituted by multiple layers offset with respect to each other to define a pair of adjacent stepped edges and a pair of adjacent reversely-stepped edges, the former intermeshing with complementary reverselystepped edges of adjoining panels, and the latter with complementary stepped edges. Each layer is composed of a Balsawood frame within which is seated a foam plastic core, the interface therebetween being packed by a compressed batting which, when the panel is subjected to cryogenic temperatures, expands to prevent gapping that otherwise would occur by reason of differential contraction of the Balsa and foam material. The reversely-stepped edges of the panel are lined with a noncompressed batting which, when the panels are interfitted, is interposed between the reversely-stepped edges of the panel and the complementary stepped edges of the adjoining panels, whereby the non-compressed batting is subjected to compression as a result of differential contraction between the wall of the container and the panels attached thereto. Each panel is preferably provided with a facing which has expansion joints formed in extended margins thereof which are connectable to the facings of adjoining panels.

United States Patent [191 Roberts et al.

1 CRYOGENIC INSULATING PANEL SYSTEM [75] Inventors: William Melchior Roberts, Blauvelt, N.Y.; George D. Dohn, Park Ridge, NJ.

[73] Assignee: Baltek Corporation, Northvale, NJ. [22] Filed; Jan. 8, 1973 [21] Appl. No.: 321,835

[52] US. Cl 52/223 R; 52/573; 52/615; 52/624; 220/9; 428/33; 428/52; 428/81 [51] Int. Cl B321) 3/10; B32b 3/14 [58] Field of Search 52/573, 615, 624; 161/37, 161/39, 40, 41, 43, 44, 161; 220/9 R, 9 LG,

9D, 9A, 9F

[56] References Cited UNITED STATES PATENTS 2,960,196 11/1960 Meserole 52/615 2,980,279 4/1961 Lueders 220/9 A Primary E.\'aminerThomas J. Herbert, Jr. Assistant Examiner-Bruce H. Hess [4 1 July 15, 1975 l 57] ABSTRACT A thermal jacket for a cryogenic container, the jacket being formed by a system of interfitting modular panels attachable to the wall of the container. Each panel is constituted by multiple layers offset with respect to each other to define a pair of adjacent stepped edges and a pair of adjacent reversely-stepped edges, the former intermeshing with complementary reverselystepped edges of adjoining panels, and the latter with complementary stepped edges. Each layer is composed of a Balsa-wood frame within which is seated a foam plastic core, the interface therebetween being packed by a compressed batting which, when the panel is subjected to cryogenic temperatures, expands to prevent gapping that otherwise would occur by reason of differential contraction of the Balsa and foam material. The reversely-stepped edges of the panel are lined with a non-compressed batting whic when the panels are interfitted, is interposed between the reversely-stepped edges of the panel and the complementary stepped edges of the adjoining panels, whereby the non-compressed batting is subjected to compression as a result of differential contraction between the wall of the container and the panels attached thereto. Each panel is preferably provided with a facing which has expansion joints formed in extended margins thereof which are connectable to the facings of adjoining panels.

16 Claims, 6 Drawing Figures CRYOGENIC INSULATING PANEL SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to thermal insulation for cryogenic purposes, and more particularly to modular panels formed of composite materials which together function to provide both structural strength and thermal insulation, the panels interfitting to create a thermal jacket enveloping a cryogenic container.

Cryogenics, which deals with the phenomena of extreme cold, is assuming considerable commercial significance. In recent years, for example, liquid gases having low boiling points, such as nitrogen and air, have been widely used to freeze perishables which are then protectively stored in insulated containers for prolonged periods without spoilage, or are transported by railroad, truck or vessel over long distances without the need for mechanical refrigeration.

It is also the current practice to liquefy natural gas or methane and to transport the gas in the liquid state in thermally-insulated tanks. The fact that natural gas in liquefied form occupies a volume which is only one sixhundredth of the fuel in the gaseous state, renders the liquefaction process economically feasible even when the liquid must be transported for thousands of miles from the oil well, where it is available as a by-product, or from a gas field to the consumer market. To this end, ocean-going vessels have been specifically fitted to carry cargoes of liquefied natural gas.

The primary concern of the present invention is with containers intended for cryogenic purposes, wherein the cargo, which may be in liquid or solid form, is at an extremely low temperature and must therefore be thermally insulated from ambient temperature. By ambient temperature" is meant the temperature of the ambient air or water to which the loaded container is exposed in storage or transit. By "cryogenic container is meant any form of thermally-insulated, lowtemperature enclosure, such as a crate or box intended for rail transportation, a thermally-insulated trailer truck, or an insulated tank to be installed on a barge, a ship, or placed in or above ground.

In all forms of cryogenic containers, the structural and thermal problems are similar, for the container must be of sufficient structural strength to support the load under the most severe conditions encountered in practice, and yet the insulation must be such as to maintain the low temperature of the load within the proper limits despite wide variations in ambient temperature. Moreover, the thermally-insulated structure must be capable of withstanding the stresses produced by the wide temperature differential between the cryogenic load temperature and ambient temperature.

The extremes of temperature to which the cryogenic container is subjected will be appreciated when it is realized that cold liquid hydrocarbons at atmospheric pressure have a temperature in the order of 265F., whereas ambient temperature may range between F. and +1 I5F. In the case of liquid nitrogen or liquid helium, the cryogenic temperature is even lower.

In the prior patents to Kohn et al. U.S. Pat. No. 3,325,037, and Lippay U.S. Pat. No. 3,298,892, there are disclosed structural panels whose core is formed of end-grain Balsa wood, the panels having an exception ally high strength-to-weight ratio as well as excellent thermal insulation properties. Balsa has outstanding properties unique in the lumber field, for it averages less than nine pounds per cubic foot, which is forty per cent of the weight of the lightest North American species. The cell structure of Balsa affords a combination of high rigidity and compressive and tensile strength, far superior to any composite, matted or synthetic ma terial of equal or higher density. Balsa has a low coefficient of expansion, and hence it deforms very little under severe temperature changes; i.e., Balsa is essentially dimensionally stable. Finally, Balsa may be processed by standard wood-working techniques.

The k-factor of Balsa is such as to render this material highly suitable as thermal insulation. k-factor is the symbol for thermal conductivity, which is the amount of heat expressed in BTU s, transmitted in I hour through I square foot of homogeneous material I inch thick, for each degree of Fahrenheit of temperature difference between opposing surfaces of the material.

If, therefore, one makes a system of thermal insulating panels to produce a jacket for a cryogenic container, and the panels are made essentially of Balsa, the cost thereof is quite high. And should one make the system with the panels composed essentially of foam plastic material, the cost would be much lower, but the structural characteristics of the system would not be acceptable.

SUMMARY OF THE INVENTION In view of the foregoing, it is the main object of this invention to provide modular panels formed of composite materials, such as Balsa wood and foam plastic, which together function to provide both structural strength and thermal insulation suitable for cryogenic applications.

More specifically, it is an object of this invention to provide modular panels of the above type which are constituted by offset layers whereby the panels are adapted to interfit with each other to create a thermal jacket that is attachable to a tank or other form of container for cryogenic liquids.

Still another object of the invention is to provide a multi-layer modular panel of the above type in which each layer is composed of a Balsa frame of high strength within which is seated a foam-plastic core whose thermal insulating characteristics are similar to those of Balsa.

A significant advantage of the invention resides in the fact that because the panel is composed essentially of layers formed of Balsa frames and foam-plastic cores, the overall insulating characteristics of the framed panel are effectively equivalent to those of a panel composed entirely of foam-plastic material, whereas the structural properties of the framed panel are far superior thereto.

Another advantage of a modular panel formed of frame layers is that the cost of the panel, which is made up in large part of low-density foam-plastic material, is substantially lower than that of a panel formed entirely of Balsa wood, even though the framed panel has structural and thermal properties comparable to those of a Balsa-wood panel.

Yet another object of the invention is to provide a modular panel composed of offset layers each having a foam-plastic core seated within a Balsa-wood frame, wherein the formation of leakage gaps between the Balsa frame and foam core as a result of differential contraction at cryogenic temperatures, is avoided by a compressed batting.

It is also an object of this invention to provide a jacket attachable to the wall of a cryogenic tank and constituted by interfitted and interconnected multilayer modular panels of the above-described type, wherein the formation of leakage gaps between the interfittcd panels as a result of differential contraction between the tank wall and the panels is obviated.

A salient feature of the invention is that the system of interfitted panels which together form a thermal insulation jacket attachable to a cryogenic container may be fabricated in a factory and thereafter readily installed at the job site. Another advantage of the invention is that the system of modular panels which form the jacket is self-sealing, and there is no need at the job site to pack the junctions of the panels.

Still another advantage of the invention lies in the fact that there are no gaps extending through the panels or through the junctions between panels that would constitute thermal leakage paths. Also, since the panel is made in a series oflayers, should a crack occur in any element of the panel the crack will not propagate through the entire panel thickness.

Briefly stated, these objects are attained in a thermal jacket for a cryogenic container formed by a system of interfitting modular panels that are attachable to the wall of the container to protectively blanket the container.

Each panel is constituted by multiple layers offset with respect to each other to define a pair of adjacent stepped edges and a pair of adjacent reversely-stepped edges, the stepped edges of the panel intermeshing with complementary reverscly-stepped edges of adjoining panels, and the reversely-stepped edges of the panel intermeshing with complementary stepped edges of adjoining panels.

Each layer of the panel is essentially composed of a frame of Balsa wood or other material having similar structural and thermal properties, within which frame is seated a core of rigid foam-plastic material. The interface between the Balsa frame and the foam core may be packed with a compressed batting which when the panel is subjected to cryogenic temperatures, expands to prevent gapping that would otherwise occur by reason of the differential contraction of the frame and foam materials.

The reversely-stepped edges of the panel are lined with a non-compressed batting which, when the panels are interfitted, is interposed between the reverselystepped edges of the panel and the complementary stepped edges of adjoining panels, whereby the noncompressed batting thereafter is subjected to compression as a result of differential contraction between the wall of the container and the panels attached thereto, thereby avoiding buckling of the panels that otherwise would occur.

The top layer of the panel may be covered by a facing sheet which serves as a vapor barrier, two adjacent margins of the sheet being extended beyond the top steps in the pair of stepped edges, and being connectable to the facing sheet of the adjoining panels. The extended margins are creased or otherwise shaped to form expansion joints to maintain the connection between panels under varying thermal conditions.

OUTLINE OF THE DRAWING For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, wherein:

FIG. I is a fragmentary perspective view of a cryogenic tank enveloped in an insulating jacket composed of a system of modular panels in accordance with the invention;

FIG. 2 is a perspective view of one of these modular panels;

FIG. 3 is a section taken along the plane indicated by line 33 in FIG. 2',

FIG. 4 is a plan view of the lowermost layer in the panel;

FIG. 5 is a section taken along the plane indicated by line 55 in FIG. 2', and

FIG. 6 is a section taken through two interfitted modular panels.

DESCRIPTION OF THE INVENTION Referring now to FIG. I, there is shown a selfsupporting tank having a wall 10 of aluminum or other suitable material, for storing a liquified gas such as methane, ethylene, propane, and chlorine. Because of the cryogenic temperatures involved, it is essential that the tank wall be enveloped in a thermal insulating jacket. For this purpose. a thermal jacket is provided which is constituted by a system of interfitting and interconnected modular panels 11 having the desired structural and thermal properties. Though the container shown in FIG. 1 is one formed by a cylindrical shell and a conical roof, it is to be understood that this is merely for purposes of illustration, for the invention is applicable to any container configuration.

The panel construction is such that the entire system of panels may be fabricated in a shop and later installed at the job site. Each panel 11 is secured to the wall 10 of the tank, hence the inner surface of the panel is subjected to extremely low temperatures, whereas the outer surface thereof is exposed to ambient temperature. The panel, therefore, must satisfy thermal and structural requirements dictated by the form and function of the container. In practice, the panels need not be attached to the tank but can be attached to each other to define a jacket surrounding the tank.

The insulation panel must be of sufficient strength to support the structural load to which it is exposed. It will be appreciated that the panels which blanket the exterior of the tank at the bottom thereof may be subjected to a heavy load depending on the nature of the tank support system, as distinguished from the side panels, which are relatively unloaded. The panels must also have thermal characteristics minimizing the transfer of heat to maintain the desired low temperature of the liquid contained in the tank within proper limits despite wide variations in ambient temperature. Moreover, the panel must be capable of withstanding the stresses produced by a wide thermal gradient within the panel.

Referring now to FIG. 2, it will be seen that each panel is composed of three layers 11A, 11B and 11C, laminated together. In practice, only two layers may be used, or a number greater than three. Obviously, the thicker the panel, the greater is the insulation provided thereby.

Layers 11A, 11B and 11C are offset with respect to each other to define a pair of adjacent stepped edges E, and E and a pair of adjacent reversely-stepped edges E and E thus making it possible for the panels in the system to be neatly interfitted without any separation therebetween.

When panels are interfitted, the stepped edges E, and E of each panel intermesh with complementary reversely-stepped edges of the adjoining panels. whereas the reversely-stepped edges E and E intermesh with the complementary stepped edges of the adjoining panels. Thus each panel forms a module or building block which is joined to four other like modules. As will be later explained. the panels are all connected to the wall of the container and to each other.

Since the container or cryogenic tank in many instances is in cylindrical or spherical form, whereas the panels shown in the drawing are flat, for a given installation the panel dimensions are made such with respect to the degree of curvature of the tank wall against which the panel lies, that there is only a slight displacement therebetween. However one can perform panels' of the type disclosed herein to impart to these panels a curved profile conforming to the curvature of the tank for which the panels are designed.

As will be later seen. each panel is bolted to the tank wall and because the Balsa-foam panel has some degree of bendability, the initial slight displacement between the curved wall and the flat panel contiguous thereto, is erased. Thus the system of attached modular panels is caused to conform to the curved profile of the tank wall and the thermal jacket presents an appearance whose form corresponds to that of the tank which it envelopes.

The bottom layer 11A of the modular panel. as shown separately in FIG. 3, is constituted by a frame formed of Balsa-wood beams 12 laid over and laminated to a somewhat broader frame 13 formed of plywood sheeting. Seated within the Balsa frame and resting on the shoulder defined by the plywood and Balsa frames is a core 14 made of rigid foam-plastic material. Packed between the four-sided interface of the Balsa frame and plastic-foam core is a compressed batting 15 formed of fiberglass or other suitable cryogenic batting or compressible packing material.

While the frame and core are shown as being of square form. in practice, the layers may have other geometric forms, such as rectangular. triangular or trapezoidal. depending on the panel layout requirements. Also while a foam plastic core is disclosed, other insulating materials, such as a fiber glass core, may be used in lieu thereof, particularly at the bottom layer.

The Balsa may be of the end-grain type, by which is meant that the grain is parallel to the load imposed thereon. ln end-grain form, Balsa has a higher compressive strength than flat-grain Balsa, for in the latter type, the load is perpendicular to the grain direction. In some applications, however, where the load is not essentially compressive in nature, the flat-grain Balsa may be the more appropriate material. The invention is not limited to frames of Balsa and one may, for example, use synthetic materials. such as solid plastics having similar structural properties and acceptable thermal characteristics.

The foam-plastic material may be constituted by polyurethane foam, polystyrene foam. polyvinyl chloride foam (PVC). or any other commercially available, low-cost insulating material having acceptable thermal properties.

Though Balsa is available in weights of approximately 6 pounds per cubic foot, which is far lighter than other forms of lumber. foam plastics are still lighter. Thus, polyurethane foam. depending on its density, is available in weights of2 to 4 pounds per cubic foot. The kfactors of Balsa and urethane foam are quite comparable at low temperatures. For example, at -260F, the k-factor of one commercial form of Balsa (6 pounds per cubic foot), is 0.11, or less, while that of urethane foam at 2 pounds per cubic foot runs from 0.11 to 0J4.

But denser urethane foams (4 pounds per cubic foot) have a poorer k-factor of 0.15 or higher. Hence. while the load-sustaining properties of foam-plastic material becomes greater as the density increases, this improvement is at the expense of thermal insulation properties, and it also raises the cost of the panel.

The batting l5 interposed between the foam plastic core 14 and the Balsa frame performs an important function. At cryogenic temperatures, if material shrinkage is encountered, it must be taken into account in panel design, particularly since, assuming a tank made of aluminum, it will be found that Balsa shrinks less than aluminum, whereas foam plastic shrinks to a greater degree than aluminum.

Though one may dimension a foam-plastic core to fit exactly within a Balsa frame under ambient temperature conditions. when this composite structure is later subjected to the extreme low temperatures encountered in cryogenic tanks, the resultant difference in the contraction of the dissimilar materials will produce a significant gap therebetween which opens up a thermal leakage path. This drawback is obviated by the compressed batting 15, for when differential contraction takes place, the batting proceeds to expand, to prevent the formation of a gap. Thus the thermal integrity of the panel layer is always maintained.

The second layer 11B, which overlies and is laminated to the first layer 11A, but is offset with respect thereto, also is composed of a Balsa frame 16 mounted on a broader plywood base frame 17, with a foamplastic core 18 fitted within the Balsa frame and a compressed batting 19 packed in the interface therebetween to avoid gapping. Because of the offset between layers 11A and 11B, there is an overlap between foam core 18 of intermediate layer 118 and the Balsa frame 12 of the bottom layer 11A. To avoid gapping between the layers, a sheet of batting 20 is interposed therebetween. Should the cores of the layers warp somewhat because of temperature differentials between the opposing faces thereof, the batting 20 will prevent an air gap by reason of such warpage and since warpage is permitted no undue stresses will be developed.

The topmost layer HC is of essentially the same construction as the other layers, and comprises a plywood base frame 21 on which is mounted a Balsa frame 22, within which is seated a foam-plastic core 23, with a batting 24 interposed between the core and Balsa frame. and a batting 25 between the underside of core 23 and the top surface of layer 11B.

Secured to the top surface of layer 11C is a vaporimpervious facing sheet 26, preferably of aluminum. As shown in FIG. 2, adjacent margins M and M of sheet 26 are extended beyond the top steps of the stepped edges E and E of the panel. These extended margins are creased or corrugated to define expansion joints 1, and J2.

Shown in FIG. 3, in connection with joint J, is a bolt 27 whose head is anchored by a plywood strip 28 laminated to the Balsa frame 22 of the top layer 11C. The

bolt passes through facing sheet 26 and a plywood holding board 29 overlying the margin, the bolt being fastened by a nut 30. When panels are installed on the tank the nut 30 is tightened, drawing down the plywood holding board 29 and thus firmly holding the adjacent panel to the tank but allowing it to move slightly due to the differential thermal contractions of the tank and panel.

A helical spring 31 placed between the plywood strip 28 and sheet 26, serves to raise the margin slightly to facilitate interfitting of the panels, in the manner shown in FIG. 6; but when the adjoining panels are intermeshed, the extended margin is riveted by pop-in rivet 32 to the facing of the adjoining panel, thereby interconnecting the panels.

The panel is attached to the tank wall by means of a stud 33 which is welded to the wall and projects therefrom, the stud being received in a well 32 bored in the Balsa frame 12 of the bottom layer A, and being fastened by a nut 35. After the panel is attached to the wall, the well is filled by a foam-plastic plug 36 to avoid thermal leakage.

As shown in FIG. 5, the exposed surfaces .of the re versely-stepped edges lined by a non-compressed batting 37, which may be formed of fiberglass. This batting, when the panels are interfitted, lies against the complementary stepped-edge of the adjoining panel. Since wall 10 of the tank, when the tank is filled with a cryogenic liquid, tends to contract to a greater extent than the Balsa material which frames the layers of the panel, the wall contraction causes the framed panels attached thereto to move closer together, thereby subjecting batting 37 to compression. In the absence of the batting-filled space the interfitted panels attached to the contracting wall would be forced against each other and caused to buckle.

Thus the attachment of the panels to the tank wall is such that when thermal expansions and contractions occur, no undue stresses are encountered in the tank wall or in the insulating panels. The marginal expansion joints between interconnected panels are such that they are self-adjusting to compensate for thermal expansions and contractions of the panel and the wall, without opening air spaces that would produce thermal leaks.

There is no need to pack the joints on the job site, for all necessary packing is incorporated in the panel structure. Also a sheet of batting 39 may be attached under the panel, as shown in FIG. 3, the batting being compressed when the panel is attached to the wall, thus filling any void which might occur due to differences in curvature between the panel and the tank.

While there has been shown and described a preferred embodiment of a thermal jacket for a cryogenic container in accordance with the invention, it will be appreciated that many changes and modifications may be made therein without departing from the essential spirit of the invention. For example, instead of having panels with stepped and inversely stepped edges to effect panel connections, tongue-and-groove and other forms of joints may be used for this purpose. Also the core of each panel layer instead of being in the form of a block of foam-plastic may be constituted by a honeycomb whose cells are filled with foam-plastic whereby the core would have improved dimensional stability and compressive strength. Also, in lieu of using precompressed batting to account for the differences in thermal contraction between the foam core and Balsa frame, this effect may be taken care of by precompressing the foam core in such a manner that during cooling, the compressive stresses in the foam are reduced rather than the foam shrinking. In this way thermal gaps that otherwise would occur are avoided.

We claim:

1. A thermal jacket for a cryogenic container, the jacket being formed by a system of interfitting modular panels, all of which are attachable to the container wall, each panel comprising:

a plurality of layers which are laminated together and are offset with respect to each other to define a pair of adjacent stepped edges and a pair of adjacent reversely-stepped edges, the stepped edges of the panels interfitting with complementary reverselystepped edges of adjoining panels in the system, and the reversely-stepped edges interfitting with complementary stepped edges of adjoining panels in the system; each layer being composed of a main frame within which is fitted a unitary core of rigid foam plastic insulation material, the frame being made of balsa wood material that contracts to a lesser degree than the foam-plastic material at eryogenic temperatures, and a compressed batting formed of thermal insulating material interposed between the core and the frame, which batting expands when the panel is subjected to cryogenic temperatures, thereby avoiding gapping.

2. A thermal jacket as set forth in claim 1, wherein said Balsa wood is of the flat-grain type.

3. A thermal jacket as set forth in claim 1, wherein each of said layers is provided with a secondary frame formed of plywood underlying the main frame, said secondary frame being broader than the main frame to create a shoulder therewith to receive the core.

4. A thermal jacket as set forth in claim 1, wherein said reversely-stepped edges are lined with noncompressed batting.

5. A thermal jacket as set forth in claim 1, further including a metal facing sheet secured to the top layer of the panel to provide a vapor-impervious barrier.

6. A thermal jacket as set forth in claim 6, wherein said facing sheet is formed of aluminum.

7. A thermal jacket as set forth in claim 5, wherein the margins of the facing sheet are extended over the reversely-stepped edges, the extended margins being creased to define expansion joints and being connectable to the facings of the adjoining panels.

8. A thermal jacket as set forth in claim 1, wherein wells are bored in the frame of the lowermost layer in the panel to receive mounting studs projecting from the wall of the container.

9. A thermal jacket as set forth in claim I, wherein said panel is formed of three layers.

10. A thermal jacket as set forth in claim 1, wherein said core is formed of a foam-plastic material whose kfactor is comparable to that of Balsa.

11. A thermal jacket as set forth in claim 10, wherein said foam plastic is polyurethane.

12. A thermal jacket as set forth in claim 10, wherein said foam plastic is polyvinyl chloride.

13. A thermal jacket as set forth in claim 10, wherein said foam-plastic is polystyrene.

14. A thermal jacket as set forth in claim 1, wherein said compressed batting is formed of fiberglass.

which is fitted a unitary core of relatively low strength rigid foam plastic insulation material. and means to join the panel to an adjoining panel, said frame being composed of a material which contracts to a lesser degree than the insulation material at cryogenic temperatures, and a compressed packing formed of thermal insulating material interposed between the core and the frame, the packing expanding when the panel is subjected to cryogenic temperatures to a degree avoiding gapping.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,894 372 DATED y 15, 1975 INVENTOHS) William Melchior Roberts and George D. Dohn it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 18 "perform" should have read preform Column 7, line 18 "32" should have read 34 line 24 "edges lined" should have read edges are lined Claim 6, line 1 "claim 6" should have read claim 5 Signed and Scaled this seventh Day of 00101901975 [SEAL] Attesr:

RUTH C. MASON C MARSHALL DANN Arresting ()jfirer Commissioner of Parents and Trademarks 

1. A thermal jacket for a cryogenic container, the jacket being formed by a system of interfitting modular panels, all of which are attachable to the container wall, each panel comprising: a plurality of layers which are laminated together and are offset with respect to each other to define a pair of adjacent stepped edges and a pair of adjacent reversely-stepped edges, the stepped edges of the panels interfitting with complementary reversely-stepped edges of adjoining panels in the system, and the reversely-stepped edges interfitting with complementary stepped edges of adjoining panels in the system; each layer being composed of a main frame within which is fitted a unitary core of rigid foam plastic insulation material, the frame being made of balsa wood material that contracts to a lesser degree than the foam-plastic material at cryogenic temperatures, and a compressed batting formed of thermal insulating material interposed between the core and the frame, which batting expands when the panel is subjected to cryogenic temperatures, thereby avoiding gapping.
 2. A thermal jacket as set forth in claim 1, wherein said Balsa wood is of the flat-grain type.
 3. A thermal jacket as set forth in claim 1, wherein each of said layers is provided with a secondary frame formed of plywood unDerlying the main frame, said secondary frame being broader than the main frame to create a shoulder therewith to receive the core.
 4. A thermal jacket as set forth in claim 1, wherein said reversely-stepped edges are lined with non-compressed batting.
 5. A thermal jacket as set forth in claim 1, further including a metal facing sheet secured to the top layer of the panel to provide a vapor-impervious barrier.
 6. A thermal jacket as set forth in claim 6, wherein said facing sheet is formed of aluminum.
 7. A thermal jacket as set forth in claim 5, wherein the margins of the facing sheet are extended over the reversely-stepped edges, the extended margins being creased to define expansion joints and being connectable to the facings of the adjoining panels.
 8. A thermal jacket as set forth in claim 1, wherein wells are bored in the frame of the lowermost layer in the panel to receive mounting studs projecting from the wall of the container.
 9. A thermal jacket as set forth in claim 1, wherein said panel is formed of three layers.
 10. A thermal jacket as set forth in claim 1, wherein said core is formed of a foam-plastic material whose k-factor is comparable to that of Balsa.
 11. A thermal jacket as set forth in claim 10, wherein said foam plastic is polyurethane.
 12. A thermal jacket as set forth in claim 10, wherein said foam plastic is polyvinyl chloride.
 13. A thermal jacket as set forth in claim 10, wherein said foam-plastic is polystyrene.
 14. A thermal jacket as set forth in claim 1, wherein said compressed batting is formed of fiberglass.
 15. A thermal jacket as set forth in claim 1, wherein said container has a curved wall and the dimensions of said panels in the system are such as to cause the panels to substantially conform to said wall.
 16. A thermal jacket for a cryogenic container, the jacket being formed by a system of interfitting modular panels, each panel comprising a plurality of layers which are offset with respect to each other and are laminated together, each layer being composed of a frame of high strength balsa wood insulation material within which is fitted a unitary core of relatively low strength rigid foam plastic insulation material, and means to join the panel to an adjoining panel, said frame being composed of a material which contracts to a lesser degree than the insulation material at cryogenic temperatures, and a compressed packing formed of thermal insulating material interposed between the core and the frame, the packing expanding when the panel is subjected to cryogenic temperatures to a degree avoiding gapping. 